We plan to examine the structure and function of IgA receptors on murine B cells, using, as a model, the IgA receptors on tumor 560 (T560), a tissue- culture adapted B lymphoma originally found in the gut-associated lymphoid tissue (GALT) of a (B10 X B10.H-2a H-4b)F1 hybrid mouse. T560 cells are IgG2akappa+, Ia+, B220+, J11d+, CD3-, CD4-, CD5-, Mac 1-, Mac 2-, non- specific esterase-. They bind bromelain-treated mouse RBC (BrMRBC) and binding is completely inhibited by phosphorycholine (PC)-chloride, indicating that the tumor is probably related to the CD5 set of murine "autoimmune" B cells though it lacks CD5. T560 stimulates in an MLR, presents antigen to T cells and secretes IL-1, IL-4 and Il-6 but not IL-2, IL-5 or TGFbeta. Two lines and several clones of T560 have been derived. T560.1 and its clones bear moderately high numbers of FcgammaRII, low numbers of IgA receptors, T560.2 and its clones bear low numbers of FcgammaRII, high numbers of IgA receptors. The IgA receptors are up- regulated by overnight culture of the cells with polymeric IgA. The IgA receptors differ from those described on murine T cells by others in several ways. They are insensitive to low pH, cross-react with IgM and with high concentrations of IgG, are unaffected by exposing the cells to IFN-alpha, -beta or -gamma, are down-regulated by activation of protein kinase C (PKC) with phorbol myristate acetate (PMA) and are sensitive to phosphatidylinositol-specific phospholipase C (PI-PLC) indicating that some, or all of them are glycosylphosphatidylinositol (GPI)-linked to the cell membrane, the first mouse Ig receptor discovered to be linked to the membrane in this way. We have prepared rat monoclonal antibodies (mAbs) that block IgA receptor activity. Our specific aims are: 1) To determine the distribution of the epitope(s) recognized by the mAbs on other normal and malignant cells by fluorescence-activated cell-sorting (FACS) and by standard procedures on fixed tissues. 2) To characterize the IgA receptors of T560, isolated by their affinity for the mAbs or for IgA. We shall determine their m.w., glycosylation, mode of assembly on the cell surface and whether they can be recovered from supernatants after exposure of the cells to PI-PLC. 3) To begin to clone the gene(s) for the IgA receptor. Several methods for obtaining the necessary probes with which to screen cDNA libraries are detailed. 4) To determine whether down-regulation of the IgA receptor by activation of PKC involves internalization of the receptor or loss of its affinity for ligand. 5) To establish whether the receptor plays a physiological role in the function of the cells.